How to import MakeHuman character in blender2.9 to activate pose mode
Did you make sure to add a skeleton/armature to the mesh in Makehuman?  Here's a couple of things that slowed me down:

In Makehuman, if you add clothes or other things, you are actually importing more than one mesh.  For instance, in the attached image, you'll notice that only the clothing is highlighted by clicking on the human.  I don't know if you can see it in the side panel or not, but &quot;Exports:Male_casualsuit&quot; is chosen, not Exports or Exports:body.  You need to make sure you know what you are choosing -- though I'm not sure that even that will get you to pose mode if you choose the mesh rather than the armature -- see point 2.



If you choose the mesh and not the armature, then pose mode is not available. Note in the above image that the mesh is chosen, but not the armature.  In order to go into pose mode, it seems you need to actually select the skeleton itself:


If I choose the skeleton I can then go into pose mode:

But, at the most basic level, you didn't indicate that you actually exported a skeleton at all, so there may not be any rigging done to start with.